1. Field of the Invention
This invention is generally an optical down converter for the mapping of received radio frequencies into an arbitrary intermediate frequency range while precluding interference between the received signals and more specifically a device for improving image rejection to improve the harmonic spurs limiting system performance.
2. Description of the Related Prior Art
Currently there are primarily two types of image rejection systems that provide significant frequency translation. These are those which use a digital phase modulator to produce a serrodyne phase modulated waveform, and those which split a received signal into two parts and then recombine them in such a way as to eliminate the unwanted image and carrier frequencies. Optical image rejection mixers that utilize the serrodyne method of frequency translation apply a sawtooth waveform to phase modulate the optical signal, thereby causing the optical frequency to shift. The achievable image rejection is limited by the number of discrete bits that can be implemented by the digital phase modulator when approximating the sawtooth waveform. This limits the image rejection to approximately 25 dB.
In the case of microwave image rejection mixers, the achievable image rejection is limited by the need for near perfect amplitude and phase control. As shown in FIG. 1, a received radio frequency)(RF) signal 124 is first divided into two signals 106 and 108 in a in-phase power divider 122, mixed in associated mixers 112 and 114 with a local oscillator input 102 (LO IN) and split into associated signals 106 and 108 that are shifted 90xc2x0 in a first 3 dB 90xc2x0 hybrid power divider 122. The output of the mixers 112 and 114 are then recombined with a 90xc2x0 phase shift between the two components in a second 3 dB 90xc2x0 hybrid power divider 126 and output as an intermediate frequency band 134. Exact amplitude and phase matching with broad band signals is nearly impossible with strictly microwave components since the frequency response of each component varies. A typical device, with a 3xc2x0 phase error and a 0.25 dB amplitude imbalance upon recombination, is limited to about 36 dB of image and carrier rejection. Further details on serrodyne frequency shifting can be found in Johnson et al., SERRODYNE OPTICAL FREQUENCY TRANSLATION WITH HIGH SIDEBAND SUPPRESSION, J. of Lightwave Tech., Vol. 6, No. 6, pg. 109, 1988.
A recently developed optical image rejection downconverter with  greater than 60 dB of image rejection was recently developed and utilizes an electronic mixer to upconvert signals into the passband of a bandpass filter followed by optical downconversion of the filtered signals into a desired output band, as shown in FIG. 2. SEE, U.S. patent application Ser. No. 09/620,0324, by Ward et al., entitled IMAGE REJECTING MICROWAVE PHOTONIC DOWNCONVERTER, Navy Case No. 79,800, filed Jul. 17, 2000. This device allows telecommunications systems to downconvert the lower sideband of densely multiplexed ultrawideband bandwidth channels into low frequency bands where conventional electronics can perform signal processing functions; has the image rejection ( greater than 60 dB) to provide unambiguous signals for direction finding applications; and exhibits efficient image rejection that should permit multi-octave microwave frequency reception and compression. In addition, that invention is intrinsically remoteable, and due to the various optical and electrical components that they may be used to construct the invention contained in this application. However, the invention in Ward et al. utilizes a first local oscillator frequencies below the original frequency of a bandpass filter that may allow harmonics from the local oscillator to convert undesired input radio frequency (RF) frequencies into spurious signals presenting the output intermediate frequency band being applied to user electronics. These harmonics can be handled by the user electronics but additional signal processing would be required. It would be preferable to prevent such harmonics from reaching the user electronics.
The object of this invention is to is to provide an apparatus for the improvement of image rejection in image rejection mixing systems while removing harmonic spurs.
Another object of this invention is to provide a device having sufficiently large image rejection capability so as to allow precise determination of frequency for direction finding applications.
These and other objectives are accomplished by the all optical image reject down converter which maps received radio frequency (RF) into an arbitrary intermediate frequency (IF) range, while precluding interference between the received signals. The invention converts a received radio frequency signal into an arbitrary intermediate frequency for use by an electronic circuit in other devices. An example of the systems ability to convert a received 9 GHz signal into a 2 GHz intermediate frequency follows. Optical light originating from a laser is divided into two paths. Light in a first path is transferred into an optical sideband by a first optical modulator (MZM1) or phase modulator (LO1=25 GHz). At the same time, light in a second path is converted into 18 GHz sidebands by a second optical modulator (MZM2). The signal is amplified and additional sidebands are generated by a received 9 GHz signal. At this point, the optical spectrum in the second path consists of many optical wavelengths. Next, the light is passed through a narrow-band optical filter that is tunable which selects the 27 GHz sideband. Ideally, the optical spectrum of the second path consists only of light at the frequency equal to the original laser frequency plus the 27 GHz signal. Finally, the filtered sideband is recombined (heterodyned) with the 25 GHz signal of path one, resulting in downconversion to a 2 GHz signal. The use of a narrow-band optical filter allows the system to select a particular sideband for use in the heterodyne downconversion. As a result, image frequencies present in the optical link are filtered out and are rejected. The image rejection of the system is a function of filter extinction.